摘要Continuously increasing global temperatures present great challenges to food security.Grain size,one of the critical components determining grain yield in rice(Oryza sativa L.),is a prime target for genetic breeding.Thus,there is an immediate need for genetic improvement in rice to maintain grain yield under heat stress.However,quantita-tive trait loci(QTLs)endowing heat stress toler-ance and grain size in rice are extremely rare.Here,we identified a novel negative regulator with pleiotropic effects,Thermo-Tolerance and grain Length 1(TTL1),from the super pan-genomic and transcriptomic data.Loss-of-function mutations in TTL1 enhanced heat tolerance,and caused an increase in grain size by coordinating cell ex-pansion and proliferation.TTL1 was shown to function as a transcriptional regulator and lo-calized to the nucleus and cell membrane.Fur-thermore,haplotype analysis showed that hapL and hapS of TTL1 were obviously correlated with variations of thermotolerance and grain size in a core collection of cultivars.Genome evolution analysis of available rice germplasms suggested that TTL1 was selected during domestication of the indica and japonica rice subspecies,but still had much breeding potential for increasing grain length and thermotolerance.These findings pro-vide insights into TTL1 as a novel potential target for the development of high-yield and thermoto-lerant rice varieties.